Transverse-Mode Fiber Laser with an All-Fiber Cavity and Fiber-Grating

January 1, 2006 / Vol. 31, No. 1 / OPTICS LETTERS 17

High-power single-polarization and single- transverse-mode fiber laser with an all-fiber cavity and fiber-grating stabilized spectrum

Chi-Hung Liu and Almantas Galvanauskas Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Avenue, Ann Arbor, Michigan 48109-2122

Victor Khitrov, Bryce Samson, Upendra Manyam, Kanishka Tankala, and David Machewirth Nufern, 7 Airport Park Road, East Granby, Connecticut 06026

Stefan Heinemann Fraunhofer USA, Center for Laser Technology, 46025 Port Street, Plymouth, Michigan 48170

Received August 5, 2005; revised manuscript received September 17, 2005; accepted September 21, 2005 Conventionally, large-mode-area (LMA) fiber lasers use free-space polarizing components to achieve linear polarization output. External components, however, significantly limit laser robustness and power scalability. We demonstrate, to the best of our knowledge, the first high-power all-fiber cavity single-polarization single-transverse-mode LMA fiber laser, without the use of free-space polarizing components. This has been achieved by using tightly coiled high-birefringence 20 ␮m core LMA fiber. The lasing spectrum at 1085 nm has been stabilized by a fiber grating, spliced at one end of a LMA fiber. Up to 405 W of single-polarization output with a polarization extinction of Ͼ19 dB with a narrow spectrum (1.9 nm FWHM) and in a single- transverse mode ͑M2 Ͻ1.1͒ has been demonstrated. The simplicity of a monolithic-cavity approach is highly beneficial for a number of applications, including the use of a fiber laser for nonlinear wavelength conversion and for coherent and spectral beam combining. © 2006 Optical Society of America OCIS codes: 060.2320, 060.2280. The rapid development of high-power large-mode- PM fiber laser designs, it does not use any free-space area (LMA) fibers has led to recent demonstrations of or in-line polarizing components or highly specialized 1–2 kW fiber lasers producing single-transverse- single-polarization fibers.8 Additionally, we demon- mode output beams.1,2 This achievement is particu- strate that 20 ␮m LMA core fiber monolithic cavities larly important due to the highly practical aspects of can include other spliced-in fiber components, such fiber laser technology, such as compatibility with as narrow-linewidth fiber Bragg gratings (FBGs) for monolithic designs, high efficiency, and high reliabil- stabilization of the laser spectrum. The absence of ity. Furthermore, fiber geometry appears to be ideally polarizing components significantly facilitates the suited for further power scaling by employing coher- power scalability of this approach. This simple and ent and spectral beam combining methods,3,4 which robust all-fiber design is particularly attractive for potentially can lead to ӷ10 kW power high beam beam combining as well as for a broad variety of quality fiber lasers. One important technical chal- other applications where single-polarization output lenge on this power scaling path is that beam com- is critical, such as nonlinear wavelength conversion bining critically requires single-polarization output using second-harmonic generation (SHG) or optical fiber lasers. So far, however, such single-polarization parametric oscillation (OPO). LMA fiber lasers have been either based on the use of Single-transverse and single-polarization mode op- free-space polarizing components5,6 or are complex eration in a highly birefringent LMA fiber can be master-oscillator–power-amplifier (MOPA) setups achieved by exploiting two processes. First, higher- with in-line polarizing and isolating components.7 order modes can be suppressed by using conventional Limitations imposed by such approaches are multi- fiber-bending-induced differential loss between fun- fold. The use of bulk components is unsuitable for damental and higher-order modes in low-NA LMA- monolithic all-fiber designs, a significant practical core fibers.9 Second, stress-induced birefringence sig- impediment. Both free-space and in-line polarization nificantly increases the effective refractive index of components at present have limited power handling one of the fundamental-mode polarizations, thus en- capacity, thus limiting the power scalability of single- abling one to achieve bending-induced loss differen- polarization fiber lasers. Here we demonstrate a tiation between different LP01 mode polarizations as monolithic-cavity single-polarization cw LMA fiber well. As shown in Ref. 10, the refractive index for the laser with output powers exceeding 400 W in a single light polarized parallel to the stress direction (i.e., po- transverse mode and a spectrally stabilized narrow- larized along the x axis in the fiber cross section pic- linewidth output beam. This approach is unique in ture in Fig. 1) is significantly increased by the photo- its simplicity: unlike other demonstrated high-power elastic effect, while for the polarization perpendicular

achieve single polarization in a single-transverse mode at the output. Three fiber-coupled pump modules operating at 915, 940, and 976 nm wavelengths were wavelength multiplexed and coupled from the straight-cleaved end. One more pump module operating at 915 nm was coupled from the angle cleaved end with a FBG. All four pump modules provided a maximum of 610 W of pump power coupled into the fiber. Fiber was heat sunk through a conductive heat- removal arrangement, which is much more efficient than convective heat removal in air and which was sufficient to eliminate thermal effects in the fiber at all used pump powers. Figure 3 shows the measured output power of the laser. With the total 610 W of coupled pump power, up to 405 W of the laser output has been achieved Fig. 1. Calculated bending loss for eigenpolarized spatial with the slope efficiency of 65.9%. Minor slope- modes of 20 ␮m and 0.06 NA core highly birefringent ͑⌬n efficiency reduction compared with that for an un- =3ϫ10−4͒ LMA fiber. A picture of fiber cross section with principal axes is also shown. coiled fiber (70.9%) indicates a minor effect of the coiling-induced extra loss of the “slow” x-polarized to the stress direction (i.e., polarized along the y axis) LP01 mode. The inset in Fig. 3 shows 1085.2 nm cen- the refractive index remains nearly unchanged. In tered laser spectra at 184 and 405 W, revealing spec- general, a lower effective refractive index corre- tral broadening from 1.2 to 1.9 nm with increasing sponds to a weaker guidance of the correspondingly output power. This broadening indicates the reduc- polarized mode and consequently to a higher suscep- tion of threshold due to the increase of gain in the Yb tibility to the bending. Therefore with a properly se- fiber, which directly relates to and increases mono- lected core refractive index and fiber-bending radius tonically with pump power. We found that, compared the x-polarized LP01 mode can remain impervious to with the spectral width of the fiber grating the bending, while higher-order modes and the ͑ϳ1.5 nm͒, the laser spectral width is still well con- y-polarized LP01 mode can experience large bending- fined by the grating, which implies the possibility of induced loss. Calculated transverse- and narrowing down the laser linewidth by using narrow polarization-mode bending-loss characteristics for spectral-width fiber gratings. 20 ␮m 0.06-NA core highly birefringent ͑⌬n=3 ϫ10−4͒ LMA fiber used in this work are shown in Fig. 1. In the experiment fiber was coiled to 7.5 cm in diameter (marked by the vertical dashed line), at which the calculated bending loss of Ͼ1 dB/m is relatively high. However, negligible reduction of the slope efficiency observed experimentally at this coiling radius indicates that the model overestimates bending- induced loss, and a tighter coiling than predicted needs to be used. The setup of the fiber laser is shown in Fig. 2. An Yb-doped, panda-type, highly birefringent LMA Fig. 2. Monolithic-cavity single-polarization LMA fiber double-clad fiber, whose cross section is shown in the laser. inset of Fig. 1, is used as the gain medium in the sys- tem. The fiber has a 20 ␮m core and 400 ␮m octagon- shaped pump cladding with 0.06 and 0.45 NA for the core and the cladding, respectively. The fiber length of 33 m has been selected to optimize pump absorp- tion at 940 nm. One end of the Yb-doped fiber was straight cleaved and the other end was spliced with a high reflectivity ͑RϾ99%͒ fiber Bragg grating at 1085 nm imprinted into an identical 20 ␮m core and 0.06 NA LMA fiber. The splicing procedure was care- fully developed to produce negligible loss and inter- modal scattering at the splice ͑Ͻ0.1 dB͒. A picture of the splice is also shown in Fig. 2. The laser cavity was formed by a straight cleaved end (3.5% Fresnel reflection) and the high-reflectivity FBG. The free fiber end on the grating side was angle cleaved with ϳ11° to eliminate end reflections. The Yb-doped fiber was coiled with 7.5 cm diameter, which allowed us to Fig. 3. Output laser power and spectrum. January 1, 2006 / Vol. 31, No. 1 / OPTICS LETTERS 19

SBS is not a real limiting factor in this configuration as long as the laser bandwidth is sufficiently broad to suppress Brillouin gain. Our estimates indicate that ⌬␭ϳ0.1 nm is sufficient to achieve this, thus permitting further significant linewidth reduction compared with our current result. In summary, we have demonstrated a simple tech- nique to achieve high-power single-polarization and single-transverse mode operation from a monolithic LMA-fiber laser cavity. The practical significance of such a laser is that it does not use any free-space or in-line polarizing or isolating components and can be built by using standard off-the-shelf Yb-doped double-clad fibers. The achieved 405 W output was limited by the available pump power. Our analysis indicates that multi-kW output powers should be Fig. 4. Polarization extinction ratio at different laser out- achievable with this approach. Measured Ͼ18.8 dB put powers. polarization extinction did not degrade with output Figure 4 shows the polarization extinction ratio power. Also, it is likely that the obtained PER value (PER) of the laser output measured at different laser is measurement limited and higher values are prac- powers. No significant power dependence has been tically achievable. A particular feature of the demon- observed. At all power levels measured the PER was strated laser is its narrow-linewidth operation, Ͼ18.8 dB. Note, however, that at Ͻ200 W output the achieved by including a narrow-linewidth fiber grat- measured PER is ϳ20 dB with an ϳ1.2 dB drop at ing into the monolithic laser cavity. Such monolithic ϳ200 W. For output powers Ͼ200 W the PER power PM and narrow-linewidth design are particularly dependence remains flat. We identified this 1.2 dB valuable for future fiber laser power scaling using drop to be caused solely by the change of measure- both coherent and spectral beam combining tech- ment setup, which was necessary at higher powers, niques, and the whole class of other applications—for and not by any abrupt change in laser behavior itself. example, high power nonlinear wavelength conver- We have also measured output beam M2 values at sion. different output power levels. At all powers measured C.-H. Liu’s e-mail address is [email protected]; V. M2 Ͻ1.1, with an observed tendency to slightly in- Khitrov’s e-mail address is [email protected]. crease with increasing output power (from M2 =1.02 References at the threshold to M2 =1.05 at the highest power). This tendency is at least partly associated with the 1. Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, Opt. increasing difficulty of accurately measuring beam Express 12, 6088 (2004). quality at higher beam powers due to an increasing 2. 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